In order to be able to operate gas turbines with improved efficiency or in order to increase power in particular during but also after start-up, it is advantageous to shift the gas turbine rotor counter to the main flow direction in the gas turbine. To that end, gas turbines preferably provide means for hydraulic gap adjustment that allow the gas turbine rotor to be shifted so as to set a smaller gap spacing between the turbine blade tips and the casing internal wall of the expansion turbine, and a corresponding larger gap spacing between the compressor blade tips and the casing internal wall of the compressor. Typically, the geometry of such gas turbines means that the losses arising in that context in the compressor are smaller than the increase in power in the expansion turbine due to the reduction in the gap spacing.
Changing the position of the gas turbine rotor in this way can normally be carried out only in a gas turbine that has been sufficiently warmed through. Otherwise, there is a risk of an undesired temporal change in the spacings between the blade tips and the casing internal wall, which can even lead to damage to the gas turbine. In this context, the gap adjustment means typically take the form of hydraulic shifting means in the region of the axial stops, and they permit precise-path displacement by means of hydraulic pistons in the region of the main and secondary tracks in the axial bearing. The means forhydraulic gap adjustment also comprise, in addition to these hydraulic shifting means, suitable closed- or open-loop control means via which a desired adjustment of the gap spacing can be undertaken.
On account of adjustment via the means for hydraulic gap adjustment, the expansion turbine can work more efficiently and can therefore achieve improved conversion of thermal energy into mechanical energy. Accordingly, this reduces the turbine outlet temperature of the exhaust gas when the turbine inlet temperature remains constant. If the gas turbine were now to be set to a constant turbine outlet temperature, it would by implication be necessary to raise the turbine inlet temperature insofar as a gap adjustment is or were to be undertaken.
In the case of operation at rated load, the consequence would be over firing of the gas turbine, with the possible consequence of damage to the first blade rows of the expansion turbine. In order to prevent such damage, prior to operation of the means for hydraulic gap adjustment, the operation of the gas turbine is always adjusted by means of an exhaust gas temperature control (ATK control) so as to reduce the turbine outlet temperature. Adjustment of that kind is normally performed by controlling the quantity of fuel that is supplied to the burner, whereby it is possible to ensure that the permissible turbine inlet temperature is not exceeded.
In that respect, the ATK control ensures that the turbine inlet temperature is set as low as possible in order to thus keep the material loading, in particular in the region of the first turbine stages, as low as possible. In addition, the ATK control also serves to make use of a maximum possible turbine inlet temperature in order to thus achieve a gas turbine efficiency that is as high as possible.
However, the disadvantage of these methods known from the prior art is that, primarily in the case of partial load, an ATK-controlled reduction in the turbine outlet temperature also simultaneously leads to an undesirable reduction in the efficiency of the gas turbine. Especially during start-up of a gas turbine, during which the turbine is in partial load operation, it is thus for example impossible to make full use of the gas turbine potential and the full efficiency.